Selective Silencing of Foreign DNA with Low GC Content by the H-NS Protein in Salmonella

Abstract

Messages are received both near and far, they show, to create transient and sustained responses. The cone presynaptic terminal is highly invaginated, with ribbons of glutamate-containing vesicles above each invagination. Cones respond to changes in light with graded changes in membrane potential. Decreases in light intensity depolarize cones and increase glutamate release, which then activates a class of cells known as Off bipolar cells. In the new report, DeVries et al. show that Off bipolar cell dendrites contact cone terminals at two sites. Most subtypes of Off bipolar cells contact the base of the cone terminal, 300 nm away from the vesicle fusion sites. The group found, however, that one Off cell subtype extended its dendrites up into each invagination to end close to fusion sites. These contacts within invaginations experienced large, rapid fl uctuations in glutamate levels when a cone was depolarized. Glutamate then spilled out of the invaginations to the basal contacts. In spite of their distance from release sites, even a single vesicle's worth of glutamate was able to reach and activate these cells. Distance exacted a toll, however, as the glutamate concentrations sensed by these cells fl uctuated more slowly and at much lower levels. The invaginating cell senses glutamate via AMPA receptors, which recover rapidly from glutamate-induced desensitization and can thus decode rapid consecutive pulses. Basal cells instead use kainate receptors, which have much slower recovery times and produce responses that average over rapid fl uctuations in glutamate concentration. The basally located Off bipolar cells thus generate more sustained responses. The steady signal conveys the basic sight information of change magnitude and duration. The transient signal saying just that there was a change "is probably very important," says DeVries, "because it can help an animal avoid predators or moving objects."

Diverse Functions of Restriction-Modification Systems in Addition to Cellular Defense

“…The transcription termination factor Rho and nucleoid-associated proteins are two such factors that are implicated in the silencing of foreign genomic elements. Nucleoid-associated proteins selectively bind to xenogeneic DNA with AT contents higher than that of the genome and silence them (45,46). The Rho protein of E. coli was recently implicated in causing the premature transcription termination of horizontally acquired genes and prophages in the genome (47).…”

“…The widespread occurrence of R-M systems among eubacteria, archaea, and certain viruses of unicellular algae (46,97,187,188) may hint at their other functions in addition to cellular defense. These additional roles could be better understood by looking into the evolutionary route by which a particular R-M system acquired a new biological function.…”

DNA supercoiling is a fundamental regulatory principle in the control of bacterial gene expression

“…This protein has a promiscuous relationship with the genome, binding to hundreds of sites that have little in common other than a high A + T base content and some intrinsic DNA flexibility (Dillon et al 2010;Lucchini et al 2006;Navarre et al 2006;Oshima et al 2006). It binds to nucleation sites on the DNA and then spreads laterally; its bridging capacity lends to it a Velcro-like quality that can zip together different segments of the chromosome Dame et al 2006;Maurer et al 2009;Noom et al 2007).…”

The regulatory role of DNA supercoiling in nucleoprotein complex assembly and genetic activity

“…In vivo, two principal functions have been ascribed to H-NS: as a repressor of certain genes and gene clusters located in A/T-rich regions of the genome (Navarre et al 2006;Ouafa et al 2012), and as a domainin, assumed to function by stabilizing barriers between distinct topological domains (Hardy and Cozzarelli 2005). Both functions are compatible with the structures assumed by plectonemically coiled DNA superhelices.…”